We report here that the heteroleptic BHT-Mg(OR) complex, i.e. [(BHT)Mg(OBn)(THF)]₂ (Mg3), represents an effective and versatile ring-opening polymerisation (ROP) catalyst for several cyclic ethylene phosphate monomers (CEPMs), such as ethylene phosphates with methoxy- (MeOEP, 1), isopropoxy- (ⁱPrOEP, 2), and tert-butoxy- (^tBuOEP, 3) substituents, N,N-diethyl ethylene phosphoramidate (Me₂NEP, 4) as well as ethyl (EtPPn, 5) and tert-butyl (^tBuPPn, 6) ethylene phosphonates. Mg3 retains its catalytic activity over a broad temperature range from −50 to 100 °C and efficiently carries out the fast and controlled polymerisation of CEPMs with sterically unhindered alkoxy groups with less than 1% chain branching even at near-complete conversion. Compared with ROP catalysed by 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), Mg3 performs much better in the polymerisation of CEPMs with bulky groups and of monomers sensitive to strong nucleophiles, such as ^tBuOEP.